Apparently this now exists!
The Space Marines of the Adeptus Astartes are the defenders of mankind, the ultimate superhuman brotherhood standing against the evils of the galaxy. But even among their hallowed ranks, there are those who would turn their backs on their brothers and follow the path of the traitor. Gathered within these pages, you will find tales from the dark side of the Imperium – remember, that which can defend, can also betray...
New York Times bestselling editor Christian Dunn presents a selection of short stories from some of Black Library’s finest authors: Anthony Reynolds, Aaron Dembski-Bowden, Matthew Farrer, Sarah Cawkwell and many more.
I'm included among the 'many more'.